In the automotive sector and in buildings, glazing systems made from compositions containing transparent thermoplastic polymers such as polycarbonate, for example, offer many advantages over conventional glazing systems made from glass, such as greater break resistance and weight savings. In the case of automotive glazing systems, they provide greater passenger safety in the event of traffic accidents, and the weight savings reduce fuel consumption. Finally, transparent thermoplastic polymers and compositions containing transparent thermoplastic polymers provide substantially greater design freedom due to their easier moldability.
However, the high diathermancy (i.e., transmittance for IR radiation) of transparent thermoplastic polymers leads to an undesirable temperature rise inside the vehicle under the influence of sunlight. As described by Parry Moon, Journal of the Franklin Institute 230, pages 583-618 (1940), most solar energy lies within the near infrared (NIR) range between 650 and 1100 nm next to the visible range of light between 400 and 750 nm. Penetrating solar radiation is absorbed inside a vehicle, for example, and emitted as long-wave heat radiation at 5 to 15 μm. Since conventional glazing materials and transparent thermoplastic polymers in particular are not transparent in this range, the heat radiation cannot dissipate to the outside. A greenhouse effect is obtained. In order to minimise this effect, the transmission of glazing systems in the NIR should be kept as low as possible. Conventional transparent thermoplastic polymers such as e.g. polycarbonate are transparent in both the visible range and in the NIR, however. Therefore additives are needed for example that demonstrate as low as possible a transparency in the NIR with as high as possible a transparency in the visible range of the spectrum.
Infrared absorbers for this purpose, which limit this temperature rise, are described in the literature (e.g. J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, 1197 (1992), U.S. Pat. No. 5,712,332, JP 06240146 A).
Dyes having absorption maxima in the near infrared (NIR) are known for such applications. For exterior applications, however, a high long-term light resistance against discoloration and fading is also necessary in addition to the required spectral properties.
A distinction is made between organic and inorganic NIR absorbers. Inorganic NIR absorbers conventionally display a high light stability but have the disadvantage of being insoluble in thermoplastics and therefore forming cloudy to opaque molding compositions. By contrast, organic NIR absorbers that are soluble in thermoplastics are known, but they display a lower light stability.
Thin, film-like, heat-absorbent multi-layer structures are described in JP 10-077360 A that contain both (A) a phthalocyanine infrared absorber and (B) an ultraviolet absorber. The weathering resistance of the thermal insulation layer is said to be improved by this means. According to one example, a film-like coating layer with a thickness of 0.13 mm is proposed, which contains both phthalocyanine infrared absorber and ultraviolet absorber. It is true that a coating layer of this type displays an acceptable weathering resistance in 48-hour accelerated weathering tests. For practical applications, however, weathering tests of more than 500 hours are necessary. Furthermore, the thin, film-like coating layers described in this publication display inadequate optical properties and are therefore unsuitable for use in automotive glazing.
In order to improve the weathering resistance of heat-absorbent multi-layer structures, a thin, film-like, heat-absorbent coating comprising 3 layers with a gradual reduction in phthalocyanine infrared absorber is also proposed in JP 10-077360 A. This contains an ultraviolet absorber in the top layer, which is directed towards the incident light radiation, a mixture of ultraviolet and phthalocyanine infrared absorbers in the middle layer and additional phthalocyanine infrared absorber in the bottom layer, which is directed away from the incident light radiation. The disadvantage of a multi-layer structure of this type is the expensive three-layer construction and the poor optical properties of the multi-layer structure, which is produced by lamination.
It is also generally known that certain thermoplastics can be protected by the use of UV-absorbent paints and/or coextruded layers having a high content of UV absorber. It is thus known from EP 0 110 221 A, for example, that the weather resistance of polycarbonate plastic sheets can be improved by coating them with a layer containing 3 to 15 wt. % of a UV absorber. The multi-layer systems described in this publication contain no infrared absorber.
Finally, EP 0 774 551 A describes heat filters based on inorganic pigments which display a UV-absorbent protective layer. Inorganic pigments have the disadvantage of not dissolving in thermoplastics, which means that moldings that are transparent in the visible range and display low haze are not obtained.